1. Technical Field of the Invention
This invention relates generally to calibration of pressure sensing equipment, and more particularly to dynamic pressure differential generation for handheld calibration of pressure measuring instruments.
2. Description of Related Art
To calibrate instruments, such as differential pressure transmitters in HVAC (Heating, Ventilation and Air Conditioning) Systems in-place, a NIST (National Institute of Standards and Technology) traceable handheld calibrator is typically used to provide an accurate reading of a pressure differential between two pressure lines. One method typically used to provide pressure to the pressure lines is with large units using a static pressure source. Typically, a static pressure is provided by compressing a closed volume of air a desired amount to obtain a higher pressure within a high pressure line. A low pressure line provides either ambient pressure or another reference pressure. A handheld calibrator is used to provide an accurate reading of the pressure differential between the two pressure lines. When a sensor in an instrument to be calibrated is exposed to the pressure differential, the readings from the instrument may be adjusted or verified to match the readings of the NIST traceable handheld module. In this way, the instrument sensor may be calibrated. A drawback associated with the use of a static pressure source is that, when measuring very small pressure differentials, e.g. 0.01xe2x80x3 WC (approximately {fraction (1/270)}th of a psi), even slight temperature changes can affect the pressure within a closed volume. Minor leaks are also a problem with closed volume systems. Therefore, it is typically difficult to generate and maintain the constant pressures over time. The inability to maintain constant pressures over time causes difficulties in calibrating instruments that require field calibration and verification. Field calibration verification in Biotech/Pharmaceutical applications is mandated by agencies, such as the FDA (Food and Drug Administration). Due at least in part to the above mentioned difficulties, many users have a very difficult time with instrument calibration.
Another type of pressure source used for instrument calibration is a dynamic pressure generator. Dynamic pressure generators are large apparatuses, typically confined to table top use in a laboratory.
A compact, portable and NIST traceable pressure source for dynamically generating very low pressures is provided. The pressure source has at least one adjustable valve and a pressure generating element in-line with a fluid, such as air. A differential pressure is created across the pressure generating element due to the resistance of flow through the element. The pressure generating element may be a valve or a laminar flow element. The differential pressure varies depending on the volume of the gas flow through the device and the amount of resistance to the gas flow. The pressure source utilizes a miniature pump to create a pressure, or, alternatively, a vacuum, that causes gas flow. The pressure source is a compact device or module and is capable of providing a very low and stable differential pressure by using a dynamic flow that compensates for temperature changes, volume changes and leaks. The compact module may be configured as a plug-in module for existing handheld calibrators for operator ease.